In a radio relay device that accommodates a radio line as one example of a line relaying device, the transmission speed that is capable of transmission varies, as it is affected by the surrounding environment. In general, when setting up a network, a network architect assumes a throughput for transmission in accordance with the transmission speed designed first and accommodates a TDM (Time Division Multiplexing) line and an Ethernet (registered trademark) line as described in Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2003-259471, “packet/TDM integrated node device”. Accordingly, when the transmission speed does not vary, transmission with the throughput assumed by the network architect is possible. However, when the transmission speed varies due to the effect of the surrounding environment and the like, it is assumed that transmission with the throughput of the accommodated TDM line and Ethernet line is not possible at the current transmission speed of a variable-transmission-speed device-connecting line, which results in a failure to provide appropriate services.
When the network topology is the ring architecture, even when the transmission speed in one route of the variable-transmission-speed device-connecting line that is connected to the line relaying device decreases due to the effect of the surrounding environment and the like, the other route is connected to the destination by the ring architecture, and it is possible to achieve transmission through a detour path using the other route.
However, although detour, throughput reduction and the like can be implemented as protocols by flow control or switching in the case of Ethernet packet communication, a fixed bandwidth is required and a transfer route needs to be uniquely determined in advance in the case of TDM communication.
Therefore, in the case where the transmission speed of the variable-transmission-speed device-connecting line that is connected to the line relaying device varies due to the effect of the surrounding environment and the like, when the transmission speed decreases to fall below the transmission bandwidth required for the TDM line as a degree of variation in the transmission speed, there is a problem that it becomes impossible to transmit the TDM line data, and the data is discarded without outputting the TDM line data from the variable-transmission-speed device-connecting line.
Further, even when modification to the radio modulation scheme or the like is made, there is a possibility that a variation in the speed of the variable-transmission-speed device-connecting line occurs, and as a result, it is only possible to transmit the TDM line data which can be transmitted with the transmission bandwidth after the variation among the TDM line data flowing into the line relaying device. Accordingly, there is a problem that, even when the radio modulation scheme is modified, the TDM line data that overflows the transmission bandwidth is not transmitted to the opposite device.